For more than ten years, bridle chains used to pull towed bodies (2500 lbs) have been been failing with increasing frequency. The chains' links are 3/4 " in diameter and composed of four different metals and welding fillers. Available documentation indicates that all failures were due to discontinuities in the weldments of chains' links that were caused by poor welding or excessive casting porosity. It was decided to reduce the stainless steels used to two: 31600 (316) and S20910 (Nitronic 50). The welding filler used to weld 316 was changed to low carbon 316 (316L). The 316 was used to fabricate the links and the S20910 used for other components requiring greater strength. My concern is that simply changing the 316's filler to low carbon 316 would not end the failures, particularly in light of the long history of failures.

Although using the 316L filler apears to resolve the potentially corrosive problems caused by sensitization when 316 is subjected to high temperatures produced by welding, there may still be long-term corrosion problems. Even though the low carbon of the 316L weldment drastically reduces the amount of chromium carbide produced by carbon pulling chromium out of solution to produce chromium carbide, the high temperature zone (HAZ) at the two interfaces of each link's 316L weldment with the 316 base material appears to be susceptible to corrosion. This is due to the dilution (diffusion) of 316 into the weldment at both interfaces. The percentage that 316 contributes to the weldment depends on the welding process used. The process being used, GMAW, contributes up to 20% of 316 to the weldment at each interface for a total dilution of 40% max. With the increase of 316 in the weldment, and the subsequent increase of chromium pulled out of solution, the weldment appears to be susceptible to corrosion and is the chain's "weak link".

Research shows that common industry practice is to use low carbon 316 (316L)whenever the metal is welded in order to avoid corrosion. In addition, it appears that most distributed 316 stainless steel has had its carbon reduced below .03% thus meeting 316's higher strength requirement and 316L's low carbon requirement and is designated as dual certified. The price is approximately the same as 316's. The problem is that 316 is being used as the links base metal rather than 316L. With industries using 316L in plain water applications, it seems that using 316 in a salt water environment is taking an unnecessary risk with corrosion especially in light of the past long term problems with 316. The bridle chain is typically used several hours three to six days a month.

The engineer making the decision explained that 316 was being used because in his experience he was unaware of any corrosion problems with 316 and that 316L was unnecessary. With the previous failures due to poor welding, using 316L would make it much easier for welders to avoid sensitization by using 316L. Using 316L, instead of 316, allows the stainless steel to remain inside the range of 1100 deg. F to 1300 deg F for hours before sensitizing 316L. On the other hand, if 316 doesn't cool down rapidly and remains in the range of 1100 deg. F to 1300 deg F for more than a minute or two, the metal becomes sensitized, and thus making the 316 stainless steel more susceptible to corrossion over time.

Research shows that standard industry practice is to use 316L whenever the metal is to be welded, yet it has been decided that 316 should be used in salt water! Has anyone had any experience with 316 or 316L in a salt water environment and what is your opinion about using 316 in such an environment?

Thanks in advance for your help.